Figure 8.

Graphene metasurfaces as active quarter-wave plates. (a) Graphene asymmetric nanocross for LP to CP conversion. The ${45}^{°}$ LP beam is converted into a CP beam at $7.92\mathsf{\mu }\mathrm{m}$ when the graphene Fermi level is 0.75 eV. The operation frequency blue shifts with the increase of the Fermi level. Reproduced with permission from [80], Copyright Optical Society of America, 2013. (b) Hybrid metasurface composed of graphene sandwich grating and gold grating separated by a polymer spacer. The form birefringence in graphene grating and gold grating adds up to offer a constant phase delay of ${90}^{°}$ between two eigenmodes over a wide bandwidth. Reproduced with permission from [81], Copyright The authors, 2015. (c) Graphene grating sandwich with well-designed distance and in-plane gradient on top of the LC layer. The spatial gradient of the graphene grating increases the form birefringence, and the electrical control of the LC molecule direction leads the QWP to have a dynamic bandwidth of $78\%$. Reproduced with permission from [82], Copyright The authors, 2018. (d) A graphene sheet patterned with butterfly holes and backed by seven layers of graphene with separate biasing for wideband LP to CP conversion. The bottom seven layers of graphene show a tunable reflection phase in order to compensate the difference of the phase delay due to the frequency shift over a wide bandwidth. Reproduced with permission from [83], Copyright The authors, 2017.